1. Field of Invention
This invention relates generally to methods and devices for measuring characteristics of an optical signal. More particularly, it relates to devices and methods for determining wavelengths of a plurality of optical communication signals.
2. Discussion of Related Art
Optical communication systems are growing with the growing demand for faster and broadband reliable networks. Wavelength division multiplexing (WDM) is one technique used to increase the capacity of optical communication systems. Such optical communication systems include, but are not limited to, telecommunication systems, cable television systems (CATV), and local area networks (LANs). An introduction to the field of Optical communications can be found in “Optical Communication Systems” by Gowar, ed. Prentice Hall, NY, 1993.
In WDM systems as well as other types of optical communication systems, it may become necessary to monitor each transmitted channel in order to determine the wavelength and power of each channel and to determine the optical noise intensity near each source wavelength. Such monitoring becomes increasingly important as one increases the number of wavelength channels transmitted through the fiber. The information produced by the channel monitor may be used for feedback control of the light source or sources, to control signal quality during environmental changes and reconfiguration of the system, to monitor instability in various components of the system, or to ensure that a selected channel is the correct one.
A limited form of wavelength monitoring is disclosed by Villeneuve et al in U.S. Pat. No. 5,825,792. Villeneuve et al disclose the use of a Fabry-Perot (FP) filter disposed within an optical path of the signal to be measured. The FP filter is positioned at an angle so that different wavelengths of light are transmitted at varying angles with respect to the incident light. A pair of photodiodes is used to provide an electric signal for feedback control of the light source.
Another approach is disclosed by Mizrahi et al in U.S. Pat. No. 6,111,681. Mizrahi et al teach a method and apparatus for providing a stabilized optical selector. The device in U.S. Pat. No. 6,111,681 correlates a wavelength selector to a wavelength emitted by an optical transmission source. The optical transmitter includes a wavelength reference which is coupled to a feedback loop. The feedback loop uses thermal control to adjust the output of the transmitter. A tap in the transmission line diverts a portion of the transmitter's energy to a wavelength reference, such a Bragg grating. A photodiode detects light transmitted through the Bragg Grating and provides a feedback signal to a microprocessor which directs the thermal control of the transmitter. Mizrahi et al. likewise disclose the reverse device, providing a feedback loop for the thermal strain control of the reflection wavelength of the Bragg grating, using the laser source as a reference.
Davis et al in U.S. Pat. No. 5,818,585 disclose a system for monitoring the reflected wavelengths from multiple strings of fiber Bragg gratings (FBG) using a scanning optical filter and an isolated duplicate reference string of FBG elements. As in the case of Mizrahi et al., the reference does not provide an absolute measurement since the FBG references themselves have a dependence on physical parameters such as temperature and strain which may vary over time.